This application relates to RF, microwave, and millimeter-wave oscillators.
Signal oscillators operating oscillating frequencies in the RF, microwave, and millimeter frequencies are widely used for in various applications. As an example, such oscillation signals at these frequencies may be used for generating, tracking, cleaning, amplifying, and distributing RF carriers. Such an oscillator may exhibit phase noise and cause the oscillating frequency to draft or fluctuate. This phase noise is undesirable and may adversely affect device performance in devices using such an oscillator.
One way to mitigate this phase noise in signal oscillators is to provide a negative feedback control loop where the feedback signal is out of phase with the oscillator signal in order to stabilize the oscillator. For example, a voltage-controlled oscillator (VCO) may be designed to use voltage controlled phase-locked loop to control the oscillator frequency by minimizing the difference between the current phase values of the oscillator signal and of another relatively delayed oscillation signal. A typical VCO is a two-port device and the phase-locked loop is connected between the two ports. VCOs may be used for, among others, clock recovery, carrier recovery, signal modulation and demodulation, and frequency synthesizing.
A negative opto-electronic feedback loop may be used as the phase-locked loop in a VCO. U.S. Pat. No. 5,204,640 to Logan describes tunable voltage controlled two-port oscillators that use feedback loops with a fiber optic delay line to provide a long delay of, e.g., one millisecond or more, in stabilizing the oscillators. Such a feedback loop is an opto-electronic loop because part of the loop is the fiber optic delay line and part of the loop is electronic circuitry. A photodetector is used to convert the optical signal in the fiber optic delay line into an electronic signal in the electronic circuitry. Also, see Logan et al. in “Stabilization of Oscillator Phase Using a Fiber-Optic Delay-Line” on pages 508–512 in 45th Annual Symposium On Frequency Control (1991).